Generally, in a working machine such as a hydraulic excavator, there are provided hydraulic actuators such as hydraulic cylinders and a hydraulic motor.
A hydraulic actuator of this type is driven by being supplied with working oil and discharge of the same oil, so during the period after operation for stopping the hydraulic actuator until actual stop of the actuator, return oil higher in pressure than the working oil supplied to the hydraulic actuator is discharged from the actuator due to the own weight of an object to be actuated and an inertia force induced by driving so far performed.
Since such return oil has heretofore been recovered into a tank, the energy of the return oil has been discarded without being utilized for a certain purpose. Particularly, in case such as making a meter-out control or in case of holding a back pressure of the hydraulic actuator, the return oil is recovered into a tank through a throttle valve or the like, so that the energy of the return oil is discarded as heat.
In an effort to solve such a problem, for example in Patent Literature 1 there is disclosed a technique such that return oil from a hydraulic actuator is conducted to a hydraulic motor which is connected to a hydraulic pump to drive the hydraulic motor, thereby utilizing the energy of the return oil as power of the hydraulic pump. More particularly, according to the technique disclosed in Patent Literature 1, in an apparatus provided with a relief valve for protecting a hydraulic circuit connected to a hydraulic actuator and also provided with a switching valve disposed in an oil passage extending between the hydraulic actuator and a hydraulic motor, the flow rate of return oil supplied from the hydraulic actuator to the hydraulic motor is adjusted in accordance with a switching operation of the switching valve, thereby preventing opening of the relief valve and regenerating, as power of the hydraulic pump, the energy of working oil so far consumed for opening the relief valve.
According to this conventional technique, however, the return oil is supplied to the hydraulic motor at a flow rate which has been set for preventing opening of the relief valve, so if the power of the hydraulic pump induced by the supply of the return oil exceeds the originally required power, the hydraulic pump will discharge more working oil than necessary, with a consequent fear of a sudden increase in driven speed of the hydraulic actuator supplied with the working oil.
In a hydraulic excavator, pressure vibration may occur in the actuator circuit due to, for example, a sudden operation of the hydraulic actuator. This pressure vibration occurs also in the hydraulic excavator which adopts such a regeneration method as is disclosed in Patent Literature 1, but no countermeasure to the pressure vibration has so far been adopted by the conventional art and hence the vibration continues for a long time, giving rise to the problem that the operability is deteriorated.    [Patent Literature 1] Japanese Patent Laid-Open Publication No. 2003-120616